About Authorization describes Google’s OAuth 2 scopes. If you’re not using Duplicati OAuth Handler option “Google Drive (full access) login” I think you’re getting "Google Drive (limited) login, which keeps apps from accessing files that are not theirs. Usually this is the safer approach, but it might be what broke your import.
Google describes these as “Per-file access to files created or opened by the app.” versus “Full, permissive scope to access all of a user’s files”. I’m not sure if there’s a good way to force Duplicati to upload your files. There might be some other workaround available, but I’m not deeply expert in how Google Drive does this…